The color switching accompanied with external bias is defined as electrochromism, where the external bias triggers either a change of molecular stack or electron transfer (redox process). Since a conjugated polymer offers the tool to tune the optical properties by structural modification, polymer electrochromics have been studied intensively. Further, a conjugated polymer which changes its color by a redox process, has high optical contrast ratio, rapid redox switching, and long-term stability have made the polymer a desired material for various electrochromic device applications.
Although various colors of the conjugated polymer at the neutral state have been shown, including colors such as blue, green, yellow, and red, only a few examples of black electrochromic polymers have been reported to date. This is due to the complexity of designing materials absorbing the entire visible region (about 390-750 nm) evenly. The donor-acceptor approach offers a low band gap polymer, which covers most of the visible region. The two distinct absorption bands, generally shown in a donor-acceptor type polymer, are adjustable by the composition of donor and acceptor unit in the polymer backbone. To absorb evenly in the visible region, randomness of the donor and acceptor distribution is needed.
The first black electrochromic polymer was prepared by using a 3,4-propylenedioxythiophene (“ProDOT”) derivative as donor and a 2,1,3-benzothiadiazole (“BTD”) derivative as acceptor. The random copolymer consisted of donor and acceptor units that were chemically polymerized to yield the polymer. The copolymer showed different absorption spectra varied by the choice of polymerization method, such as FeCl3 mediated polymerization or Pd-catalyzed cross coupling, and the scale of the reaction. However, the composition and randomness of ProDOT and BTD in the conjugated polymer is unchangeable, since the two aromatics are chemically bonded.
The second approach is the electrochemical polymerization of two donor-acceptor types of monomers, one covering blue and red, and the other covering green in the visible region. The resulting conjugated polymer exhibits successful color transition from black to grey, but the method holds the complexity of control of the composition via electrochemical copolymerization and the difficulty to achieve mass production.
There remains a need in the art for improved methods, in terms of convenience and efficiency, to prepare a series of black electrochromic polymers.